1. Field of the Invention
The invention relates to power storage devices (e.g., secondary batteries) having electrolyte layers.
2. Description of the Related Art
Secondary batteries are widely used as the power source for various mobile electronic devices including laptop computers and mobile phones, and secondary batteries are also used as the power source for the power plant of electric vehicles, or the like. With regard to such secondary batteries, various technologies have been proposed to suppress the deterioration of the battery performance due to the variation of the thermal conditions (For example, refer to Japanese patent application publication No. 2006-173095 (JP-A-2006-173095) (Para. 0008 to 0011, Para. 0015-0022, FIG. 1, etc) and Japanese patent application publication No. 2005-340089 (JP-A-2005-340089) (Para. 0029 to 0032, FIG. 3, etc)).
JP-A-2006-173095 describes a battery unit constituted of a plurality of battery cells stacked on top of each other. The heat radiation of this battery unit is improved by setting the lengths of the short sides of each electrode, the area of the electrode, and the thickness of the battery unit so as to satisfy a given relational expression.
JP-A-2005-340089 describes a bipolar battery in which, in order to improve the heat radiation, at least one of the thickness of a positive active material layer and the thickness of a negative active material layer is made smaller than the thickness of the separators.
However, the structure of the battery unit described in JP-A-2006-173095 does not address the issue of the heat radiation of each battery cell. Therefore, the heat radiation from each battery cell may be insufficient and it may make the temperature distribution in each battery cell uneven.
Further, in the bipolar battery described in JP-A-2005-340089, the thickness of the active material layers and the thickness of the separators are uniform, the temperature distribution in a plane perpendicular to the direction in which the components of the bipolar battery are stacked (will be referred to as “stacking direction) is uneven. That is, in a bipolar battery constituted of a plurality of bipolar electrodes stacked with electrolyte layers interposed in-between, there exist regions having different heat radiations in planes perpendicular to the stacking direction. For example, in some cases, the heat radiation at the region located at the center in a plane perpendicular to the stacking direction is lower than the heat radiation at the region at the periphery in the same plane.
In such bipolar batteries, if the separators, etc., have a uniform thickness, it is difficult to minimize the unevenness of the temperature distribution due to the heat radiation differences mentioned above.